Stem cell injections into the heart are actively being pursued as a potential therapy for myocardial infarction and heart failure. While the ongoing trials with adult-derived stem cells show moderate clinical benefits, significant progress in the field is expected to arise from the use of cardiomyocytes derived from induced pluripotent stem cells. Despite great promise, eventual clinical use of pluripotent stem cell-derived cardiomyocytes faces a number of challenges that need to be resolved including key issues with inadequate cell maturation, phenotypic heterogeneity, arrhythmogenesis, low viability after implantation, and scale-up. Therefore, in this project we aim to establish a novel approach for cardiac cell and gene therapy that does not rely on the use of stem cells. Instead we propose to employ in vitro or in situ genetic engineering of fibroblasts into electrically active cells with customizable electrical phenotype that can couple with surrounding cardiomyocytes and improve their electrical and contractile function. Specifically, in Aim 1 we will utilize minimum st of genetic manipulations to rapidly and efficiently convert adult human fibroblasts into a readily expandable and homogeneous source of excitable cells that autonomously fire and conduct action potentials. In Aim 2, engineered fibroblasts with select electrophysiological phenotypes will be characterized for their functional interactions with neonatal rat cardiomyocytes in well-controlled in vitro co- culture systems. In Aim 3, we will establish if contractile function of infarcted rat hearts can be improved by implantation of engineered excitable fibroblasts or retroviral conversion of endogenous fibroblasts into electrically active cells. In addition to abov experimental studies, we will utilize computer simulations to facilitate genetic engineering of excitable cells and enhance mechanistic understanding of their functional interactions with native cardiomyocytes in vitro and in vivo. We believe that the proposed genetic and tissue engineering approach will provide strong foundation for the future experimental and clinical use of engineered fibroblasts in cell- and gene-based therapies for cardiac infarction and arrhythmias.